It takes $b$ minutes for Sylvia's bathtub to fill with water. How many minutes will it take to fill $3$ same-size bathtubs? Write your answer as an expression.
Explanation: Let's see what happens as the time it takes to fill the bathtub increases: Time it takes to fill the bathtub Total minutes ${1}$ $3 \cdot {1} = 3$ ${2}$ $3 \cdot {2} = 6$ ${3}$ $3 \cdot {3} = 9$ Time it takes to fill the bathtub Total minutes ${b}$ $3 \cdot {b} = 3b$ The answer: $3b$